Ann Gibbons

When it comes to weight loss, a calorie is a calorie; a kilojoule is a kilojoule. That has been the mantra of nutritionists, dietitians, and food regulators for more than a century. But when it comes to comparing raw food with cooked food, or beans with breakfast cereals, that thinking may be incorrect. That is the consensus of a panel of researchers in Boston who list the many ways the maths doesn't always add up correctly on food labels.

In a wide-ranging discussion of how food is digested in everything from humans to rats to pythons, the panel reviewed a new spate of studies showing foods were processed differently as they moved from our gullet to our guts and beyond. They agreed net caloric counts for many foods were flawed because they did not take into account the energy used to digest food; the bite oral and gut bacteria took out of various foods; or the properties of different foods themselves that sped up or slowed down their journey through the intestines, such as whether they were cooked or resistant to digestion.

The process used to estimate calories for food was developed at the turn of the 20th century by Wilbur Atwater. It was a simple system of calculating four calories for each gram of protein, nine calories for each gram of fat, and four calories for each gram of carbohydrate (modified later by others to add two calories for a gram of fibre).

Although it has been useful for estimating the energetic costs of metabolising many foods, its shortcomings have been known for decades - and some nations, such as Australia, have dropped the system because it is ''inaccurate and impractical,'' says panelist Geoffrey Livesey, a nutritional biochemist and director of Independent Nutrition Logic in Britain.

One key area where the system is inaccurate, Wrangham reports, is in estimating the calories for cooked food. Cooked items often are listed as having more calories than raw items, yet the process of cooking meat gelatinises the collagen protein in meat, making it easier to chew and digest - so it takes fewer calories to eat. Heat also denatures the proteins in vegetables such as sweet potatoes, says Harvard University evolutionary biologist Rachel Carmody, who studies the energetics of digestion.

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The way foods are processed can also make them easier to digest. Take ''resistant'' starch in cereal kernels, such as barley grain, or beans, which take a long time to digest. Grind the same cereals into flour or process it into breakfast cereal or instant oatmeal, and it becomes easy to digest, says biochemist nutritionist Klaus Englyst of Englyst Carbohydrates, a carbohydrate chemistry firm in Southampton, in Britain. This is why ''bread is more rapidly digested; beans more slowly,'' he says.

New studies also are finding that bacteria in the gut respond differently to processed foods and cooked foods. Carmody reports she and Peter Turnbaugh of Harvard University are finding ''key differences in the type of bacterial communities'' in the guts of mice, depending on whether they are fed chow or cooked meat.

''The food you eat has an enormous impact on the gut bacteria,'' and, in turn, on the energetics of digestion, Carmody says.

Why does all of this matter? Because the West is in the midst of an obesity epidemic and counting calories has been misleading, says David Ludwig, a paediatric endocrinologist at Boston Children's Hospital and Harvard Medical School. How the body processes different foods matters. ''The quality of calories is as important as the quantity of calories.''

While others welcome applying ''the best science'' to the problem of weight loss, they provide a word of caution about getting too worried about precise calorie counts.

''You can put a tonne of effort into getting more accurate calorie counts,'' says nutrition scientist Christopher Gardner of Stanford University. ''But why are you doing this? Will it make a real difference? If you want to lose weight, you still have to cut back on calories.''

A few calories here or there may not matter to most people. But to the panel members, every little bit counts.